The evaluation of parameter effects on cefoperazone treatability with new generation anodes

In this study it was aimed to investigate the treatability of cefoperazone with new generation Sb-doped SnO2-Ni anodes. For this purpose, it was studied with Sn/Sb/Ni: 500/8/1 anodes for the oxidation of aqueous solution containing cefoperazone antibiotic by addition of different types of electrolyte. Potassium chloride was found as the best electrolyte type affecting the electrochemical reactions positively even at lower concentrations (750 mg/L−1). At pH 8 the best results were obtained, which is the neutral pH value of the aqueous solution. 50 mA/cm2 was found as the best value for current density parameter, providing full mineralization just after 60 min of reaction. The removal efficiencies increased generally with the increase of current density, because active oxidants occur increasingly at higher current values. According to the results of the study it was seen that, electrochemical oxidation processes with Sn/Sb/Ni–Ti anodes could be carried out efficiently without need adding extra electrolyte (salt) and pH adjustment step for real wastewaters containing antibiotics. Thus, it was found an easy and economic way to perform electrochemical oxidation with Sn/Sb/Ni–Ti anodes for the wastewaters containing cefoperazone antibiotics.

. It was observed that the current efficiency increases when the nickel ratio increases. At the same time, it is clearly seen that the current efficiency increases with the increase of number of coatings. Thus, it was decided to study with 500/8/1:S/Sb/Ni ratio. In Fig. 2 it is seen the effect of the number of coatings on the current efficiency in Nickel-plated (Sn/Sb/ Ni) anodes.
Determination of electrolyte type. At Fig. 3a, b it is seen the effect of electrolyte type on chemical oxygen demand (COD) removal from the water samples with Sn:Sb:Ni:500:8:1 anode. It was evaluated the effect of electrolyte type (NaCl and KCl) on electrochemical oxidation process with Sn:Sb:Ni:500:8:1 anode at various electrolyte concentrations. While the electrolyte type (NaCl, KCl) and concentration affected the removal efficiencies positively by increasing the conductivity and occuring chlorine gas and hypochlorid acid as efficient oxidants, addition of extra electrolyte may increase the process cost 34 .
The COD concentrations decreased to the zero after 60 min of anodic oxidation with 2000 mg L −1 NaCl addition, at 50 mA cm −2 current density and pH 8. However, COD concentrations decreased to the zero just after 60 min with 750 mg L −1 KCl.. Although high efficiencies were obtained in a short time for 2000 mg L −1 and 2500 mg L −1 NaCl conc., 750 mg L −1 KCl was regarded as the best electrolyte type and concentration due to the excess use of salt may increase the cost and chemical consumption. Yonar et al. (2019) investigated electrochemical color removal from organized industrial district (OID) wastewater treatment plants using new generation Sn/ Sb/Ni-Ti:500/8/0.5 anodes and they studied with NaCl as the electrolyte at the range of 250-1000 mg/L. However, they found that, there was no need to addition of salt due to the study with real wastewater samples containing domestic wastewater 35 . Buyukada (2018) investigated TiO 2 assisted photocatalytic ozonation of leather effluent and it was reported that increase in both of ozone and catalyst doses caused to increase in removals of chemical oxygen demand and turbidity 36      Effect of potassium chloride concentration. At Fig. 4 it is shown the effect of potassium chloride on total organic carbon (TOC) and residual CFP removal with Sn:Sb:Ni: 500:8:1 anode. The effect of KCl (potassium chloride) concentration (mg L −1 ) on TOC and CFP were evaluated between 250 and 1000 mg L −1 KCl doses, at pH 8 and 50 mA cm −2 current density. TOC was completely mineralized within 60 min reaction with 750 mg L −1 KCl as it was stated in a graph in Fig. 3. In the same way, residual CFP was decayed more efficiently just in 5 min with 750 mg L −1 KCl. Furthermore, after 30 min oxidation with 1000 mg L −1 KCl, the complete mineralization was achieved. However, the excess use of salt may increase the cost and chemical consumption. Thus, 750 mg L −1 was determined as the best dose for the electrochemical oxidation of cefoperazone with Sn:Sb:Ni: 500:8:1 anode. Sivrioğlu and Yonar (2016) investigated the removal of COD and color from textile wastewater with using Sn/Sb/Ni-Ti anode. Although the better efficiency was obtained at high NaCl concentrations, 1 g/L NaCl concentration was chosen as the best salt concentrationto prevent the high cost and environmental problems. While all the antibiotic compounds were completely removed with NH 3 ·H 2 O-NH 4 Cl within 2 h, the removal efficiency was found below 80% with the addition of Na 2 HPO 4 -NaH 2 PO 4 , and after a 2 h of reaction time with Na 2 SO 4 , 82.4%, 83.6% and 88.4% removal rates were obtained for tetracycline, oxytetracycline and chlortetracycline, respectively 41 . Fig. 5 it is shown the effect of pH on chemical oxygen demand (COD), total organic carbon (TOC) and residual cefoperazone (CFP) decay. At pH 8, the best removal efficiencies were obtained. Just after 60 min of reaction, COD and TOC was consumed completely, and the residual CFP was consumed just in 5 min at pH 8. Thus, according to the graphs in Fig. 4, pH 8 was found as the best pH value for the electrochemical oxidation of cefoperazone. According to these results, it was assumed that, it is possible to save costs by working at neutral pH value (pH 8) of water containing cefoperazone antibiotic, because the process doesn't require extra labor and chemical costs to adjust the pH. Yonar et al. (2019) investigated electrochemical color removal from industrial wastewater with using new generation Sn/Sb/Ni-Ti: 500/8/0.5 anodes between pH 3 and 9. After 30 min of electrochemical reaction, COD and color removal efficiencies reached up to 98% and 99%, respectively at pH 8.2 (T: 25 °C) for colored industrial wastewater 35 . In a study made by Zakaria and Christensen (2014), it was reported that 100% removal efficiency was achieved within 5 min for Reactiv Blue 50 dye at a concentration of 1000 mg L −1 at pH 4.1, with using Sn/Sb/Ni anode and platinized titanium cathode fed to the membrane electrochemical electrode reactor system 42 . Abbasi, Soleymani, and Parsa (2015) performed an ozonation process on Rhodamine B molecules in aqueous solution using a reversible electrochemical ozone generator system with a titanium-based anode coated with nanocomposite Sn/Sb/Ni and they saw that the degradation efficiency could reach up to 99.5% at pH 3.7 for a 8 mg/L dye solution after 30 min 43 .

Effect of pH. At
In the experimental studies of Sivrioğlu and Yonar 44 , COD and color removal was found as 98% and 99%, respectively as a result of electrochemical oxidation of dyeing wastewater at pH 3. However, although the natural pH value (pH 7.2) of the wastewater showed relatively lower efficiency (3%) compared to the acidic conditions, pH 7.2 was identified as the best in order to avoid extra pH adjustment step and chemical cost. At alkaline www.nature.com/scientificreports/ pH values, the potential of chlorine gas and hypochlorite ion formation could support the removal of organic compounds 45,46 . Yao et al. saw that, the increase of initial pH had a great effect on ammonia removal from dyeing wastewater with Ti/PbO 2 anode and the best pH value was found as 8.3 39 . In a study of Kaur et al. (2018), with a Ti/RuO 2 anode, removal of ofloxacin (OFL) occurred rapidly in the first 15 min of the reaction when the initial pH of the synthetic wastewater was between 2 and 9, and after 30 min, there was no any significant change. It was found that higher removal efficiencies were observed with lower pH values and after only 30 min of electro-oxidation 88.6% OFL degradation was observed at pH 2, while 68.6% OFL degradation was obtained within 30 min at pH 9 47 . Hai et al. (2020) investigated the removal of sulfamethoxazole (SMX) with boron-doped diamond anode at pH 3, pH 7 and pH 11 at a current density of 30 mA cm −2 . It was observed that higher removal efficiencies were obtained at neutral pH (pH 7) compared to those at pH 3 and pH 11 40 .
Effect of current density. The current density has an active role in reaction kinetics and thus, affects the electrochemical reactions significantly 46 . The best current density value was found as 50 mA cm −2 due to obtainhigher removal efficiencies in shorter times. In the aqueous solutions, active oxidants occur increasingly at higher current density values thus, the removal efficiencies increased generally with the increase of current density in this study. doped Ti anodes. They reported that the degradation of antibiotics within a reaction of 2 h without power supply was less than 2% indicating that the adsorption of antibiotics on the anode surface could be negligibly low.
With the increase of current density, the removal rates of tetracyclines gradually increased and the removal rates were 98.1%, 97.6% and 99.5% for tetracycline (TC), oxytetracycline (OTC) and chlortetracycline (CTC), respectively at a current density of 15 mA/cm 2 for 2 h. However, the removal rates of tetracycline, oxytetracycline and chlortetracycline were found to be 79.5%, 82.0% and 90.3%, respectively, at a current density of 5 mA/cm 241 . In Fig. 6 it is shown the effect of current density (10-50 mA cm −2 ) on total organic carbon (TOC) and residual CFP removal with Sn:Sb:Ni: 500:8:1 anode (pH 8 and KCl conc.: 750 mg L −1 ).

Evaluation of total intermediate product formation. Total intermediate product formations are
shown in Fig. 7 according to the effect of KCl concentration, pH and current density.According to the graphs in A number of intermediate products were identified by using HPLC and LC-MS. N atoms in LEV were released as NH +4 and NO −3 ions. Nitrogen atoms mainly transformed into NH +4 rather than NO −3 . The concentration of NH +4 reached to 0.28 mM after 300 min, while that of NO −3 reached to the zero after 300 min. The nitrogen loss could be explained by the formation of volatile nitrogen compounds and the presence of oxamic acid that is hardly oxidizable by hydroxil radicals 50 . The biggest advantage of the electrochemical oxidation processes is that, pollutants are completely oxidized ideally. However, it is known that, organic compounds with high permanence are more concentrated with phase change instead of oxidation completely in classical processes.Thus, electrochemical oxidation processes are highly promising 26 .

SEM-EDS, XRD and HpAFM analyzes.
To image and to perform qualitative and quantitative analyzes of the anode as a final product and to identify phases, crystallinity, and structures of it SEM-EDS, hpAFM and XRD analyzes were made out. In Fig. 8a, b  www.nature.com/scientificreports/ 500/8/1 anodes (clean and used anodes, respectively). The developments in nanocomposite material applications in engineering (mechanical, optical, electrical and magnetic applications) are promising 51 . Thus, electrochemical oxidation processes with nanocomposites are promising with their easy applicability and low energy requirements 52,53 . The weight and atomic percentages and the peak intensities in Fig. 9a, b were given in Table 1 for the anode characterization. The peaks of the elements (Sn, Ni, Sb and Ti) were identified by the analysis of anode material. Coating on the anodes (Ni/Sb-SnO 2 ) was thin to enough detection of Ti underlying. Typical SEM micrographs of the anode intersections (assuming thicker coating than strands) 30 at × 150 magnification are shown in Fig. 8 for used (contaminated) and unused (clean) anode, respectively. It was observed that the anode materials showed a cracked morphology because of the coating process for clean anodes as stated in other studies 54 that was occurred by thermal shocking which is seen generally while cooling suddenly after taking of the anodes from oven. Cracked morphology is seen in thicker anodes mostly with large splits having three dimensional (3D) view 55 . However in contaminated (used) anodes, a smoother surface was observed that resulting from the coating of surface area with ions (carbon) and salts (Fe 3 (PO 4 ) 2 (OH) 2 ) passing from the solution. Christensen et al. (2013) investigated the effect of Ni and Sb oxide precursors and composition of the anode in ozone production with 1.0 M HClO 4 30 . Typical SEM images of the anodes (93.3, 6.0 and 0.7% Sn, Sb and Ni respectively) which were taken from the intersection (× 5000 magnification) showed cracked morphology (sources from thermal shock during the cooling suddenly after withdrawing of anodes from oven), while the coating on strands showed a smoother morphology assuming have thinner coating than the intersection. With EDS spectra of the anode it was seen that, the peak with 4.52 eV may be sourced from Ti underlying that could be derived from thinner catalyst coating than the strands. Christensen et al. (2012) studied the effect of Ni/ Sb-SnO 2 loading on electrocatalyst and they observed with the SEM images that the electrode was thicker and had very little pores having typical "cracked morphology" with deep crevices 55 . Moreover, it was observed that Ni/Sb-SnO 2 coating was thin sufficiently to able to detect Ti element underlying. Zhi et al. (2017) investigated the degradation of tetracycline antibiotics with Ti/SnO 2 -Sb anodes and used the sol-gel technique to coat the anode 56 . Although there were some cracks ranging in size from 1 to 10 μm, SEM images revealed that the anode surface was generally solid and smooth. In addition, they concluded that the formation of these cracks could  ) reported that the Ti substrate surface pretreated with Ti/SnO 2 -Sb 2 O 3 /PbO 2 anode is irregular and crusted, which is presumed to be formed as a result of oxalic acid application 41 . Thus, they stated that it may be beneficial to add SnO 2 -Sb 2 O 3 and PbO 2 as interlayers and active layers, respectively. In addition, Figs. 10 and 11 show AFM images of the anodes showing the topographical height change during the electrochemical oxidation process. It has been observed that there is a very high difference between contaminated and clean anode in terms of topographic height. While topographic height differences tend to increase in parallel for upper cross and bottom cross section for the clean anode; irregularities were observed for contaminated anode, which is thought to be due to ion transfer from the aqueous solution. However, it is known that the physicochemical properties of the anodes are directly related to the preparation methods. Composition ratios, particle size, surface structure, specific surface area and bonding force directly affect the performance of the anode 57 . Figure 12 shows the XRD results of clean and contaminated Sn/Sb/Ni-Ti anodes. Consequently, XRD results confirmed other SEM-EDS findings. It was observed that the surface of the used (contaminated) anode was filled with other ions (carbon) and salts (Fe 3 (PO 4 ) 2 (OH) 2 ) may source from the solution. It was worked a total of 300 h with the anodes, and thus, it was obtained that the anode material is not corroded significantly.

Conclusions
In this study, electrochemical oxidation of aqueous solution containing antibiotics with using new generation Sn/Sb/Ni: 500/8/1 anodes were carried out. pH, electrolyte type and conc. and current density parameter effects and total intermediate product formation was evaluated.
KCl was found as the best electrolyte type affecting the elctrochemical reactions positively the most even in lower concenrations. Thus, according to the results of the study, it could be possible to obtain higher removal efficiencies with real water/wastewater samples (assuming include KCl ions, mostly) without additon extra chemicals as the electrolyte. pH 8 is the neutral value of the aqueous solution containing antibiotics was obtained as the best due to provide higher removal efficiencies. Therefore, it could be possible to operate process easier and more economically by working at neutral pH values, due to there is no need to additional chemical cost for extra pH  www.nature.com/scientificreports/ arrangement step. The removal efficiencies increased generally with the increase of current density, because of the occuring of active oxidants increasingly at higher values. 50 mA/cm 2 was found as the best for current density, having full mineralization after 60 min. In our study, it was obtained more efficient results at lower current densities (50 mA/cm 2 ) with Sn/Sb/Ni anode, compared to the most of the researches on electrochemical treatment of antibiotics. According to the results of the study, it is thought that the electrochemical oxidation processes could be carried out in real wastewaters without need to adding extra salt and pH arrangement step. So, it is easy and economical way to perform electrochemical treatment processes with Sn/Sb/Ni-Ti anodes with very high removal percentages for wastewaters containing antibiotics. Also, it needs less reaction time than the conventional treatment methods. At this respect, working with these anodes is promising for the future studies. In contrast to the other materials used for anode production, Sb-doped SnO 2 -Ni anodes don't have much more toxicity and instability causing to the high costs. Additionally, these new generation anodes show very promising results in ozone production. However, most of the studies have focused on fluoroquinolone, trimethoprim, sulfonamide and macrolide for the removal of them from aquatic environments, while, just a little of them have been made for cefoperazone antibiotic. There are just only a few studies made about cefoperazone. The fact that there is no such studies ontreatment of these antibiotics with these new generation anodes has made this study unique.